1. Field of the Invention
The present invention generally concerns radiation, or optical, memory apparatus that use active, radiation-sensitive, memory media, and methods of using such apparatus and media.
The present invention particularly concerns (i) a three dimensional volume of that contains a number of separate, but interrelated, radiation-sensitive photochromic chemicals, (ii) the individually selective alteration, and the unique interrogation, of each of multiple photochromic chemicals that are located in the same addressable physical domains of the volume by use of intersecting beams of radiation having selected frequencies, and corresponding energies; (iii) the manner of using selected intersecting radiation beams at times upon each of several different photochromic chemicals held in a three-dimensional matrix, and the physical and/or chemical effects of such use; and (iv) the construction of binary-stated informational memory stores, three-dimensional patterns, and/or three-dimensional displays wherein a number of binary bits of information are stored in each of a great multiplicity of addressable physical spaces.
2. Description of the Prior Art
The present invention will be seen to concern the storage of multiple bits of information in the same physical spaces, preferably in the addressable domains of a three-dimensional (3-D) volume radiation memory, but permissively also in the addressable domains of a layers (of which there are typically one, but more than one such layer is possible) of a substantially planar radiation memory, typically an optical disc. In a 3-D volume radiation memory, the domains are addressed in order to be written and read by intersecting beams of radiation, typically by light and more typically by laser light.
In a four-dimensional (4-D) volume radiation memory, the domains are still addressed in order to be written and read by intersecting beams of radiation, but the beams are typically (i) quite short in time, or pulses, and are (ii) phased relative to one another. The xe2x80x9cfourthxe2x80x9d dimension that is referred to in the title of the present invention is thus timexe2x80x94as is taught in a related patent application to be a suitable basis, a xe2x80x9cfourth dimensionxe2x80x9d so to speak, of in the addressing a three-dimensional, volume, radiation memory store. The storage of multiple bits of information in the same physical spaces in accordance with the present invention will also be seen to also be applicable to such a 4-D volume radiation memory.
The present invention will further be seen to radiatively store, and read, information that is stored in different molecules that are co-located in the same physically addressable spaces, called domains. The writing of the information into a particular type of molecule will be seen to transpire at an associated radiation frequency, or xe2x80x9ccolorxe2x80x9d. The interrogation of the written information from the same molecules will likewise seen to transpire at an appropriate radiation Frequency, or xe2x80x9ccolorxe2x80x9d. It will be seen that, commonly, but one radiation frequency is used to interrogate all the different molecules at the same time, and that each different type of molecule will produce radiationxe2x80x94fluorescencexe2x80x94of an associated frequency, or color, in response to its interrogation.
Not only are multiple bits of information is co-located as multiple colors within the same physical domains, but all the radiative writing and reading of this information will be seen to be accomplished without contamination of, or degradation to, or confusing parallel-readout of, the information that is stored (as multiple colors, and upon multiple types of molecules) in adjacent spaces. This selectivity will be seen to be realized by (i) use of the process of two-photon (2-P) absorption in judicious combination with (ii) groups of multiple selected photochromic chemicals each of which chemicals permits that it should be (i) individually uniquely radiatively selected relative to all other chemicals, while (ii) producing no radiationxe2x80x94particularly during fluorescence upon readoutxe2x80x94that causes any change(s) in these other chemicals.
The present invention can therefore be considered to be a photochromic chemical system, or a radiation memory system using multiple photochromic chemicals, or as some combination of these aspects and attributes. The relevance of the prior art to the present invention is therefore best assessed not merely in snippets as may concern, for example, the storage of information as colors (e.g., photography), or the detection of information in multiple colors (e.g., spectroscopy), but as to how such prior art might contribute to any coherent scheme for the colored radiative reading and writing of information as colors within a unitary physical medium. With this consideration in mind, prior art concerning both (i) optical discs, and (ii) volume radiation (optical) memories, may usefully be regarded.
2.1 Previous Optical Discs, and Disc Systems
In optical disc storage, a focused laser beam writes bits on a spinning disc either (i) once and for all, or (ii) time and again. The tiny diameter of a diffraction-limited, focused infrared laser spot permits very high recording densities. Currently, re-writable optical disc drives use near-infrared lasers, with a 780-mm wavelength, to store up to 2 GB on each 5.25-inch disc.
Visible laser beams will do still better. With, say, a red beam emitting at 640 nm, capacities as high as 3 GB can fit on a single 5.25-inch disc, while a blue beam (415 nm) could pack about 5 GB into the same area.
An optical drive provides, in a sense, infinite storage. Extra room is easily acquired, and at modest extra cost, by simply adding media cartridges. Such abilities are welcome in modern computer applications.
In the basic optical drive configuration, the output of a semiconductor laser diode is first collimated by a lens, and is then given a cylindrical shape by a prism-like component called a circularizer. The collimated and shaped beam is then transmitted to a turning (45-degree) mirror, which reflects it onto a objective lens. The lens focuses the beam onto a diffraction-limited spot on the spinning optical discxe2x80x94the equivalent of a phonograph stylus. The laser stylus is used at low power to read out recorded marks, and to ensure track-following and focusing-servo functions.
The objective lens that focuses the spot is mounted on a platform, called an actuator, which moves across the diameter of the disc. Thus the actuator gives the laser beam access to any data tack on the disc.
A prime distinction between drives is which components are mounted on the actuator. A single-head optics design has all its optics mounted on the platform. In a split optics design, however, most of the optical system is fixed to the drive chassis, with only the objective lens and turning mirror being mounted on the moving actuator. The main benefit of the latter design is that the actuator weighs less, and can thus move faster and give faster access to the data.
A more crucial distinction between systems is how they record marks. The technique which is used determines the drive""s design and the type of media that can be used. In current 5.25 inch commercial systems, the marks are made in a heated medium with one of three processes: ablative (hole burning), thermo-magnetic, or phase-change. In all these techniques, the optical drivers laser is first pulsed at high power so as to heat the disc medium in preparation for recording.
In ablative recording, the focused high-power laser spot burns holes in the medium. The permanency of this way of recording data is reflected in its name: write-once, read-many (WORM) recording.
WORM recording provides the highest level of data security available in a removal device, suiting it to many applications in government, legal, and financial data archiving. In contrast, thermo-magnetic (better known as magneto-optical) and phase-change recording are re-writable processes.
In magneto-optical recording, the energy in the laser beam merely heats a spot on the disc past the disc material""s Curie point (about 200xc2x0 C.). Each magnetic domain in the hot spotxe2x80x94or rather, each domain""s direction of polarizationxe2x80x94becomes susceptible to the influence of an external magnetic field. When the material cools below its Curie point, the direction of polarization is frozen and thus data is recorded. Obviously, this type of recording is reversible, with over a million overwrite cycles possible.
Marks recorded in this way can be read out by laser because of the Polar Kerr effectxe2x80x94the fact that the optical polarization of a linearly polarized beam is rotated very slightly (0.5 degree) when reflected by a magnetic domain. The direction of rotation,depending as it does on the direction of polarization of the magnetic domain, represents the binary data recorded on the disc. With the use of error-correcting codes, the design of the read optics and electronics provides a sufficient signal-to-noise ratio to ensure an error rate of less than one in 1012.
In phase-change media, the recording material is an alloy having two phases with different optical properties, such as TeSeSn, a chalcogenide of tellurium, selenium, and tin, which has a crystalline phase and an amorphous phase.
To form a mark, a spot of crystalline recording material is momentarily melted by a laser pulse. Then the spot cools quickly into an amorphous-phase mark, whose reflectivity differs from the surrounding, crystalline-phase material.
Recorded marks can be erased through an annealing process,during which a continuous erase beam heats the material to just below its melting point, returning the allow to its crystalline state.
The simplicity of phase-change technology gives it an edge over the magneto-optical drives, which have more complicated optical paths and electronics. Another plus is that a phase-change drive, having no need for a bias magnet (or data recording, can be made thinner and consume less power. Relative disadvantages include the medium""s fewer proven overwrite cycles (about 50000) and the rate at which the material can change phase, which constrains the data-writing rate.
Both magneto-optical and phase-change systems are available in 1995. But drives of one type do not work with media of the other type. The incompatibility has been a source of confusion to users and a present damper on market growth for both technologies.
CD""s are 120-mm-diameter optical media that have recently caught on for computing. Of the three typesxe2x80x94read only, erasable, and in betweenxe2x80x94the CD ROM is the most popular. It is a read-only disc into which pits are permanently stamped to record data, much as vinyl records (or audio compact discs), for that matter) are stamped out from a master recording. To read the data on the disc, a laser stylus is shone on its surface, and the intensity of the tight reflected reveals the presence or absence of a pit.
A CD ROM can store a good deal of data (650 MB is typical), but reading and writing data is generally quite slow. Even the latest and fastest drives (6xc3x97-speed) have a data transfer rate of only 900 Kb/s, and their seek time is measured in the hundreds of milliseconds.
CD ROM drives are read-only devices and are without question the most successful of the optical drives, with unit sales doubling every year. At present, traditional optical drives do not read CD-ROM format discs, though no insurmountable technical issues prevent this.
In fact, a CD-recordable (CD-R) drive may be described as a WORM drive for CDs. An organic polymer on which marks are recorded by means of a phase-change process, the CD-R media can be played in normal CD-ROM drives. Conversely, CD-R drives can read CDs, but in addition read and write on CD-R media. Though fairly new, CD-R drives are winning a following as their prices plunge (their street price has dropped from about $5000 in 1993 to $3500 as of March, 1994).
The CD-R drive is useful for making masters for CD-ROM manufacturing, and the discs can be used for distributing a lot of information to a small audience. But because the drives record in CD format, their performance characteristics are unattractive for data storage. The Kodak(copyright) Photo CD is a type of CD-R in which photographic images are digitized and stored on the CD disc. (Kodak(copyright) is a registered trademark of the Eastman Kodak Company, Rochester, N.Y.).
On the horizon circa 1995 are CD-erasable (CD-E) systems, which one-up the recordable CD by being re-writable. There are no CD-E drives on the market at present (1995), but some are expected soon that, like CD-R, will use phase-change media.
Multi-layer storage is another idea whose time may have come. In May of 1994, IBM demonstrated an optical disc containing multiple writable layers. Because of the depth of focus of the objective lens is very small, layers can be read separately without crows talk. Moving from one layer to another requires only a slight shift of the objective lens. IBM""s disc has four (4) layers, but in theory ten (10) are possible. Multi-layer technology could expand bit capacity with the least technical risk of any approach. Before products with multi-layer capability are introduced, however, industry wide interchange standards need to be in place.
The relevance of previous optical disk technology to the present invention will be seen to be that the present invention permits recording to be done within the volume, and not merely on the surface, of a new-composition optical Compact Disc (CD). Moreover, although the recording within the volume, or thickness, of the CD is likely done in layers (which layers may, however, be on the slant as well as parallel to the plane of the disk), the xe2x80x9clayersxe2x80x9d are not physically differentiated, the volume of the disc presenting a continuum of recordable medium.
Further in assessing the relevance of previous optical disk technology to the present invention, remember that the present invention will show that multiple bits of information may be stored in the same physical spacesxe2x80x94including in the volume of a CDxe2x80x94in multiple photochromic chemicals. Recall also that, although the multiple photochromic chemicals are separately radiatively written, they are read in common so as to produce multiple frequency (i.e., different color) radiations (i.e., fluorescent lights). The present invention will show that different-color readout fluorescent light beams are separable by a monochromator as simple as a prism, and, once separated, the information within each beam is separately detectable in parallel. Accordingly, at least the laser light readout of a present-day optical CD is already similar, and somewhat compatible, with the volume CDs and volume optical memory readout systems of the present invention, requiring only slight changes in the optical readout paths, and the replication of detectors.
2.2 Three- and Four-Dimensional Radiation Memories
A three-dimensional (3-D) radiation memory store has long been recognized to be potentially useful for providing desirable characteristics not present in today""s magnetic and electro-optic memory devices. However, such a 3-D radiation memory store is, as of 1995, an immature technology. The major advantages of a 3-D radiation memory potentially include (i) random access, (ii) parallel access to considerable information per read or write cycle, typically  greater than 106 bits, (iii) high density and immense storage capacity, typically  greater than  greater than 1013 bits/cm3, (iv) very fast reading and writing speeds, typically  less than 10xe2x88x929 seconds, (v) small size, typically ≈1 cm3, (vi) low cost, projected to be  less than $103/terabyte, (vii) the absence of mechanical or moving parts, and (viii) high reliability with low maintenance and long life.
To realize these advantages, a 3-D radiation memory would desirably use (i) a long-lived and stable (photochromic chemical) storage medium, having (ii) a high writing sensitivity, (iii) a high reading sensitivity and reading efficiency (i.e., light output), and (iv) only minimal (radiative) interactionxe2x80x94crosstalkxe2x80x94between adjacent bits. Notably, the previous sentencexe2x80x94absent the words in parenthesisxe2x80x94may be considered to be equally accurate in enumerating the challenges that have previously historically beset other computer memory media and computer memory systemsxe2x80x94most notably square loop ferrite core magnetic memoriesxe2x80x94as the sentience is presently accurate in enumerating the present challenges to realizing practical 3-D radiation memories. It may thus be contemplated that each new generation of recording media of any type or any nature faces challenges of somewhat the same order; capacity, density, sensitivity, selectivity, permanence and cost.
The problems that beset different read-write-erasable optical memory media circa 1995 are several. These problems are of relatively greater and lessor severity, and impact, to the realization of various particular 3-D radiation memories, and to the practical realization of such memories. However, requirement (iv)xe2x80x94minimal radiative interaction, or crosstalk, between adjacent bits during operation and non-operation of the memoryxe2x80x94is an important requirement for 3-D memories. This requirement is particularly useful of being considered when the scheme of the present invention is evaluated. The present invention will later be seen to employ a large number of differing frequency (i.e., different color) radiations (i.e., lights) to interact with (i.e, to probe with laser light) multiple photochromic chemicals, including for purposes of causing the multiple photochromic chemicals to produce multiple frequency (i.e., different color) radiations (i.e., fluorescent lights) during reading. These many, and these differing, radiation fluxes (lights) would initially seem to be a recipe for disaster. However, the present invention will be seen to xe2x80x9csort everything outxe2x80x9d so that changes are radiatively induced only when and where wanted. Moreover, during normal operation of a memory system in accordance with the present invention, and during normal use of the photochromic chemicals of the memory store of such a memory system, no xe2x80x9cgrayingxe2x80x9d nor xe2x80x9cwashing outxe2x80x9d of the photochromic chemicals, nor xe2x80x9cany crosstalkxe2x80x9d between the information that the photochromic chemicals contain, will occur.
At the onset, this is an important area to be regarded in the prior art as hereinafter discussed. Some of this art actually teaches such chemical interaction within a volume memory store as would be anathema to the present invention.
Moreover, a color memory store where everything is stable, reversible, and non-interfering is useful as a three-dimensional display, and for three-dimensional video and television. If the action of the several photochromic chemicals are to be visually observed asxe2x80x94for example as three-dimensional televisionxe2x80x94then the contents of one portion of the volume xe2x80x9cscreenxe2x80x9d must not vary (i.e., fade, pulsate, etc.) as other portions of the xe2x80x9cscreenxe2x80x9d volume change, or come to contain differing information (i.e., different images, and image colors), during usage and over protracted time and. Like the phosphors of a quality television cathode ray tube, the photochromic chemicals of a memory or display in accordance with the present invention will be seen not to contaminate each other either chemically or radiatively or by any other mechanism, and to remain xe2x80x9ccrispxe2x80x9d and xe2x80x9ccleanxe2x80x9d indefinitely.
Previous attempts to realize long-lived high-figure-of-merit read-write-reversible volume radiation/optical memories are reviewed in the following sections 2.2.1 through 2.2.4. The sections include (i) a general discussion optical recording media, (ii) a most ancient class of three-dimensional memory stores wherein photochemical changesxe2x80x94as contrasted to photo-isomerization, or molecular excitationxe2x80x94changes occur within the memory (iii) three-dimensional (3-D) and four-dimensional (4-D) two-photon (2-P) radiation (optical) memories in accordance with the related patent applications, and (iv) a proposed figure of merit for radiation (optical) memories.
2.2.1 Optical Recording Media for, and the Use Thereof in, Two-Dimensional Optical Memories
At the present two general types of optical recording media exist, namely phase recording media and amplitude recording media. Recording on the media of the first type is based on light-induced changes of the index of refraction (i.e., phase holograms). Recording on the media of the second type is based on photo-induced changes in the absorption coefficient (i.e., hole burning).
Volume information storage is a particularly attractive concept. In a two dimensional memory the theoretical storage density (proportional to 1/wavelength xcex2) is 1xc3x971011 bits/cm2 for xcex=266 nm. However in a 3-D memory the theoretical storage density is 5xc3x971016 bits/cm3. Thus the advantages of 3-D data storage versus previous two dimensional information storage media become apparent.
Volume information storage has previously been implemented by holographic recording in phase recording media. Reference F. S. Chen, J. T. LaMacchia and D. B. Fraser, Appl. Phys. Lett., 13, 223 (1968); T. K. Gaylord, Optical Spectra, 6, 25 (1972); and L. d""Auria, J. P. Huignard, C. Slezak and E. Spitz, Appl. Opt., 13, 808 (1974).
The present invention will be seen to implement volume writable-readable-erasable optical storage in each of multiple phase recording medium each of which is also, coincidentally, an amplitude recording medium.
One early patent dealing with three-dimensional amplitude-recording optical storage is U.S. Ser. No. 3,508,208 for an OPTICAL ORGANIC MEMORY DEVICE to Duguay and Rentzepis, said Rentzepis being the selfsame inventor of the present invention. Duguay and Rentzepis disclose an optical memory device including a two-photon fluorescent medium which has been solidified (e.g., frozen or dispersed in a stable matrix, normally a polymer). Information is written into a selected region of the medium when a pair of picosecond pulses are made to be both (i) temporally coincident and (ii) spatially overlapping within the selected region. The temporally-coincident spatially-overlapping pulses create, by process of two-photon absorption, organic free radicals which store the information at an energy level intermediate between a fluorescent energy level and a ground state energy level. The free radicals store the desired information for but a short time, and until they recombine. The information may be read out by interrogating the medium with a second pair of coincident and overlapping picosecond pulses. In the case where the medium is frozen solid, interrogation may also be accomplished by directing a collimated infrared light beam into the selected region, thereby causing that region to liquefy and permitting its contained free radicals to undergo recombination. In each of the aforementioned cases, the interrogation beam causes the interrogated region to selectively fluoresce in accordance with the presence, or absence, or free radicals. The emitted radiation is sensed by an appropriate light detector as an indication of the informational contents of the interrogated region.
This early optical memory of Duguay and Rentzepis recognizes only that two-photon absorption should be used to produce excited states (e.g., singlet, doublet or triplet states) of an radiation-sensitive medium over the ground state of such medium. These excited states are metastable. For example, one preferred fluorescent medium is excitable from ground to a singlet state by process of two-photon absorption occurring in about 10xe2x88x9215 second. The excited medium will remain in the singlet state for about 10xe2x88x928 second before fluorescing and assuming a metastable triplet state. This metastable state represents information storage. Alas, this metastable state will spontaneously decay to the ground state by fluorescence after about 1 second (depending on temperature). The memory is thus unstable to hold information for periods longer than about 1 second. It should be understood that the fluorescent medium of the Duguay and Rentzepis memory is at all times the identical molecular material, and simply assumes various excited energy states in response to irradiation.
Another previous optical system for accomplishing the volume storage of information, and for other purposes, is described in the related series of U.S. Pat. Nos. 4,078,229; 4,288,861; 4,333,165; 4,466,080; and 4,471,470 to Swainson, et al. and assigned to Formigraphic Engine Corporation. The Swainson, et al. patents are variously concerned with three-dimensional systems and media for optically producing three-dimensional inhomogeneity patterns. The optically-produced 3-D inhomogeneity patterns may exhibit (i) controlled refractive index distributions, (ii) complex patterns and shapes, or (iii) physio-chemical inhomogeneities for storing data. The Swainson, et al. patents generally show that some sort of chemical reaction between two or more reactive components may be radiatively induced at selected cell sites of a 3-D medium in order to produce a somewhat stable, changed, state at these selected sites.
U.S. Pat. Ser. No. 4,471,470, in particular, describes a METHOD AND MEDIA FOR ACCESSING DATA IN THREE-DIMENSIONS. Two intersecting beams of radiation are each matched to a selected optical property or properties of the active media. In one embodiment of the method and media, called by Swainson, et al. xe2x80x9cClass I systems,xe2x80x9d two radiation beams generate an active region in the medium by simultaneous illumination. In order to do so, two different light-reactive chemical components are typically incorporated within the medium. Both components are radiation sensitive, but to different spectral regions. The two radiation beams intersecting in a selected region each produce, in parallel, an associated chemical product. When two products are simultaneously present in a selected intersection region then these products chemically react to form a desired sensible object. The sensible object may represent a binary bit of information. One or both of the radiation-induced chemical products desirably undergoes a rapid reverse reaction upon appropriate irradiation in order to avoid interference effects, and in order to permit the three-dimensional media to be repetitively stored.
In other embodiment of the Swainson et al. method and media, called xe2x80x9cClass II systems,xe2x80x9d one of the radiation beams must act on a component of the medium before the medium will thereafter be responsive to the other radiation beam. The class I and class II systems thus differ by being respectively responsive to the effects of simultaneously, and sequentially, induced photoreactions.
The Swainson, et al. patentsxe2x80x94including those patents that are riot directed to information storage and that are alternatively directed to making optical elements exhibiting inhomogeneity in their refractive index, or to making physical shapes and patternsxe2x80x94are directed to inducing changes in a bulk media by impinging directed beams of electromagnetic radiation, typically laser light, in order that selected sites within the bulk media may undergo a chemical reaction. There are a large number of photosensitive substances that are known to undergo changes in the presence of light radiation. The changed states of these substances are, in many cases, chemically reactive. The patents of Swainson, et al. describe a great number of these photosensitive and photo-reactive substances. Such substances may generally be identified from a search of the literature.
Swainson, et al. also recognize that molecular excitation from a ground state to an excited state may occur following a stepwise absorption of two photons. Swainson, et al. call this xe2x80x9ctwo-photon absorption.xe2x80x9d Swainson, et al. describe that a solution of 8xe2x80x2 allyl-6xe2x80x2 nitro-1, 3, 3-trimethylspiro(2xe2x80x2 N-1-benzopyran-2xe2x80x2-2-induline) in benzene may be exposed to intersecting synchronized pulsed ruby laser beams transmitted through an UV elimination filter to form, at the region of intersection, a spot of color. The process of stepwise absorption of two photons in this solution, and in others, is recognized by Swainson, et al., only as regards its use to produce an excited state that may form (as in the example) colored products, or that may serve as an energy transfer agent.
In making all manner of excited statesxe2x80x94including singlet, doublet, triplet, and quartet statesxe2x80x94the patents of Swainson, et al. describe known photochemistry. Generally chemistry, and photochemistry, that is known to work in one dimension is equally applicable in three dimensions. For example, it is known that an electron may be knocked off an active substance so that it becomes an ion. For example, it is known that radiation may cause a substance to dissociate a proton, again becoming an ion. For example, it is even known how to induce spin changes and changes in parity by electromagnetic radiation. Once these changes, or others, are induced in an radiation-sensitive medium then Swainson, et al. describe a reliance on the transport capabilities of the liquid or gaseous support media in order to permit a chemical reaction to transpire.
The present and related inventions reject the Swainson, et al. approach of inducing chemical reactions in a 3-D medium by creating one or more reagents by use of radiation. One reason why the present and related inventions do so is because the same support medium, or matrix, that offers those transport capabilities that are absolutely necessary to permit the chemical reactions to occur will also permit, at least over time, undesired migration of reagents or reaction products in three dimensions, destroying the integrity of the inhomogeneity pattern (i.e., the information stored).
2.2.2 A First Two Related Predecessor Patent Applicationsxe2x80x94Each for a Three-Dimensional Two-Photon (3-D 2-P) Optical Memory
The inventions of two chronologically-earliest related patent applications contemplate (i) addressing, and (ii) writing data to or reading data from, selected domains within a three-dimensional volume of radiation-sensitive medium by and with two selectively chosen, coincident, radiation beams. The radiation beams are selectively guided to spatial and temporal coincidence so as to cause certain selected domains, and only those certain selected domains, to selectively undergo selected changes by process of two-photon absorption.
The first related patent applicationxe2x80x94Ser. No. 342,978 filed Apr. 25, 1989 issued Dec. 7, 1993 as U.S. Pat. No. 5,268,862 for a THREE-DIMENSIONAL OPTICAL MEMORYxe2x80x94particularly teaches selectively inducing isomeric changes in the molecular isomeric form of selected regions within a three-dimensional radiation-sensitive medium by the process of two-photon absorption.
The method of the related invention produces a three-dimensional inhomogeneity pattern in a volume of active media in response to directed electromagnetic radiation. In order to do so, an radiation-sensitive medium having at least two isomeric molecular forms is contained within a volume. The radiation-sensitive medium is responsive to energy level changes stimulated by electromagnetic energy to change from one of its isomeric molecular forms to another of its isomeric molecular forms. A selected portion of the radiation-sensitive medium is selectively radiated with plural directed beams of electromagnetic radiation to change the selected portion from the one isomeric molecular form to the other isomeric molecular form by process of plural-photon absorption. The induced isomeric changes possess useful optical, chemical, and/or physical characteristics.
In the preferred embodiment of the first predecessor application an radiation-sensitive medium typically a photochromic material and more typically spirobenzopyran, was maintained in a three-dimensional matrix, typically of polymer, and illuminated in selected regions by two UV laser light beams, typically of 532 nm. and 1064 nm. wavelength. The illumination causes the radiation-sensitive, photochromic, spirobenzopyran medium to change from a first, spiropyran, to a second, merocyanine, stable molecular isomeric form by process of two-photon absorption. Regions not temporally and spatially simultaneously illuminated were unchanged. Later illumination of the selected regions by two green-red laser light beams, typically of 1064 nm wavelength each, caused only the second, merocyanine, isomeric form to fluoresce. This fluorescence was detectable by photodetectors as stored binary data. The three-dimensional memory can be erased by heat, or by infrared radiation, typically 2.12 microns wavelength. Use of other medium permit the three-dimensional patterning of three-dimensional forms, such as polystyrene polymer solids patterned from liquid styrene monomer. Three-dimensional displays, or other inhomogeneity patterns, can also be created.
The present application will be seen to use, as just one suitable radiation-sensitive, photochromic, medium the exact same medium as did the volume optical memory of the earliest predecessor invention: spirobenzopyran. Moreover, the present invention will be seen to make use of the variation of fluorescence between the two stable isomeric molecular forms of the spirobenzopyran moleculexe2x80x94just as did the earliest (but not all the intermediary) related patent applications.
Meanwhile, the second related patent applicationxe2x80x94Ser. No. 586,456 filed Sep. 21, 1990 for a THREE-DIMENSIONAL OPTICAL MEMORY, now issued as U.S. Pat. No 5,325,324 on Jun. 28, 1994xe2x80x94particularly deals with a system and method for addressing a three-dimensional radiation memory with two radiation beams so as to, at separate times, write binary data to, and to read binary, data from, such memory by process of two-photon absorption. The radiation beams are typically, but not necessarily, light, and are more typically laser light. Accordingly, the complete device, or system, incorporating such a volume memory was called a two-photon three-dimensional optical memory, or a 2-P 3-D OM.
The addressing of the volume memory within the 2-P 3-D OM preferably (but not necessarily) used, as a part of one component (a holographic dynamic focusing lens, or HDFL), a hologram. Thus the 2-P 3-D OM was preferably holographically addressed.
The optical memory of the present invention will be seen to dispense with the requirement for a HDFL, or for holographic addressing.
In the 2-P 3-D OM of the second predecessor application one directed beam of electromagnetic radiation was spatially encoded as an nxc3x97n wavefront array of binary bits by use of a two-dimensional spatial light modulator (2-D SLM). This spatially-encoded beam, and an additional, orthogonal, beam of electromagnetic radiation, were then selectively guided into spatial and temporal coincidence at a selected nxc3x97n planar array of domains within a three-dimensional matrix of such domains within a three-dimensional volume of radiation-sensitive medium.
This function of the spatial light modulator of the second predecessor application to spatially encode information upon a planar wavefront, or pulse, of radiation will be seen to be continued in certain color optical memories of the present invention.
In the second predecessor invention, the spatially-encoded selectively-guided coincident radiation beams served, dependent upon their combined energies, to either write (change) or read (interrogate) the condition, and particularly the isomeric molecular form, of the selected domains by a process of two-photon absorption. Remaining, un-selected, domains received insufficient (i) intensity from either beam, or (ii) combined energy from both beams, so as to be substantially affected.
This function, and property, of two-photon (2-P) absorption will also be seen to be preserved in the optical memories of the present invention.
In its preferred embodiment, the optical memory of the second predecessor application served to temporally and spatially simultaneously illuminate by two radiation beamsxe2x80x94normally laser light beams in various combinations of wavelengths 532 nm and 1024 nmxe2x80x94certain selected domainsxe2x80x94normally 103xc3x97103 such domains arrayed in a planexe2x80x94within a three-dimensional (3-D) volume of radiation-sensitive mediumxe2x80x94typically 1 cm3 of spirobenzopyran containing 102 such planes. This selective illumination served, dependent upon the particular combination of illuminating light, to either write binary data to, or read binary data from, the selected domains by process of two-photon (2-P) absorption. Cne laser light beam was preferably directed to illuminate all domains of the selected plane in and by a one-dimensional spatial light modular, 1D SLM). The other laser light beam was first spatially encoded with binary information by 2-D SLM, and was then also directed to illuminate the domains of the selected plane. Direction of the binary-amplitude-encoded spatially-encoded light beam was preferably by focusing, preferably in and by a holographic dynamic focusing lens (HDFL). During writing, the selected, simultaneously illuminated, domains changed in their isomeric molecular form by process of 2-P absorption. During reading the selected domains fluoresced dependent upon their individually pre-established, written, states. The domains"" fluorescence was focused by the HDFL, and by other optical elements including a polarizer and polarizing beam splitter, to a 103xc3x97103 detector array. The I/O bandwidth to each cm3 of radiation-sensitive medium was on the order of 1 Gbit/sec to 1 Tbit/sec.
The present invention is usable with 2-P 3-D optical memories of the forms taught within both the first and the second related applicationsxe2x80x94namely 2-P 3-D optical memories that are respectively addressed without, and with, focusing of incident radiation, particularly by focusing in a holographic dynamic focusing lens.
2.2.3 A Third Related Predecessor Patent application for a Four-Dimensional Two-Photon (4-D 2-P) Optical Memory
A third related patent applicationxe2x80x94U.S. Ser. No. AAA,AAA filed on May 30, 1995xe2x80x94is itself a continuation of a U.S. patent application Ser. No. 163,907 filed on Dec. 6, 1993. Both these two applications for a TWO-PHOTON FOUR-DIMENSIONAL OPTICAL MEMORY to inventors including the selfsame Peter M. Rentzepis who is the inventor in the present application.
A 2-P 4-D radiation memory in accordance with the third related application stores binary information in a three-dimensional (3-D) volume of a medium that is sensitive to radiation in its absorption band so as to undergo an anomalous, stable, change in its index of refraction. For simplicity, this medium is called a xe2x80x9cradiation-sensitive mediumxe2x80x9d. The 3-D volume may be, for example, in the shape of a cube of typical size 1 cm3. The cube may typically contain as a radiation-sensitive medium the photochromic chemical spirobenzopyran stably held in position in and by a matrix of, for example, polymer plastic.
The cubical 3-D volume of radiation sensitive medium is, accordance with the present invention, simultaneously momentarily radiatively illuminated along each of two axis that are mutually intersecting at a predetermined angle. The illumination axis are typically intersecting either at 90 greater than xe2x80x94perpendicularlyxe2x80x94or at 180 greater than xe2x80x94in which case the two axis are really but one axis along which the two radiation illuminations are counter-propagating.
The illuminating radiation along at least one, and preferably both, of the intersecting axis is in the form of momentary pulse. The pulse can be in the form of a plane wave, alternatively called a planar wave front. At least one illuminating radiation pulse is commonly in this form, and both pulses are preferably in this form. Both pulses must be in this form particularly in the counter-propagating intersection geometry.
The momentary, pulse, illumination serves to define and to address and to select, in a manner to be explained, a unique multiplicity of domainsxe2x80x94for example 4xc3x97106 such domainsxe2x80x94out of a very great multiplicity of such domainsxe2x80x94for example out of 8xc3x97109 such domains that are three-dimensionally (3-D) arrayed within the volume. Each selected multiplicity of domains is substantially two-dimensional, meaning that it is but one domain xe2x80x9cthickxe2x80x9d in the direction of each, and of both, illuminations. The selected multiplicity of domains can be, but need not be, in a plane (irrespective of whether either or both illuminating radiation pulses in a planar wavefront). Note that the domains are defined by the illumination, and do not represent initially, or permanently, physically or otherwise-differentiated regions within the volume of radiation-sensitive medium, which is substantially homogeneous.
Each domain stores binary information as one of two stable states, each of which states has a different associated index of refraction.
Domains within the 3-D volume of radiation-sensitive medium are written (changed from a first to a second state and associated index of refraction) by process of two-photon absorption. Domains are so written upon such times as two time-resolved radiation beams, or radiation pulses, together having a joint energy that aggregates a predetermined first energy level both (i) temporally and (ii) spatially intersect within the domain. (Remember always that the energy of a radiation beam, or radiation pulse, is a function of its frequency (E=hxcexd), and not of either its intensity or its duration.) The radiation-sensitive medium within each intersection domain is responsive to radiation of this first energy level to change from a first one of its two stable states to its other, second, state.
The responsiveness of the radiation-medium to so change is a function of the well-known quantum-mechanical equations of two-photon interaction, particularly (in the case of writing the medium) two-photon absorption. The necessity, in accordance with the law of physics, of having a temporal, as well as a spatial overlap between the two intersecting radiation pulses will prove important to the present invention. Namely, in accordance with the present invention the (i) durations and (ii) time sequence (relative phase) of the illuminating pulses, as well as their spatial directions, will be positively controlled. The size and locations of the domains, as well as the nature of changes in the state and the percentage completeness of such changes within the volume of the domain, is a function of the time (i) duration and (ii) sequence of the illuminating radiation pulses, as well as the direction of such pulses. Indeed, the directions, or axis, of both illuminating radiation pulses are normally maintained constant, for example at 90xc2x0 or 180xc2x0, for a particular embodiment of the invention. Moreover, there is no selective focusing, nor any other attempt to manipulate the spatial direction or density distribution of the illuminating radiation pulses. Each pulse typically impinges as a plane wave parallel to a face of a cubical volume straight against a face of the cube, and passes straight through the entire volume. Pulse timing is the all-important determinate of where changes (in the event of writing or erasing), or detections (in the event of reading), transpire within the volume of radiation-sensitive medium.
This scheme is considerably different than most, if not all, previous optical volume memory addressing schemes where radiation beams were simply directionally concentrated upon the volume portions where changes were to be made (possibly even by process of two-photon absorption), but where the illuminating beams were not positively temporally (as well as spatially) controlled. In accordance with the related invention, the (directionally) intersecting beams are further positively controlled both (i) duration, and (ii) time relationship (alternatively called time sequence, or phase). When the radiation illumination of the volume of radiation-sensitive medium is controlled so as to be in the form of time-sequenced radiation pulsesxe2x80x94as it is in the 2-P 4-D memory of the related inventionxe2x80x94undesired changes outside the addressed domains of the volume will, for all practical purposes, become physically impossible. Accordingly, the 2-P 4-D memory has a high figure of merit for selectivity, and considerable resistance to cumulative contamination during repetitive use.
Continuing with the write operation, if the radiation-sensitive medium at the addressed domains is already in its second state then it is unchanged by radiation of this first energy level. The radiation-sensitive medium is insensitive to the energy of either write radiation pulse taken alone, and nowhere within the entire 3-D volume is the radiation-sensitive medium changed in the slightest by either write radiation pulse taken individually.
Each domain within the 3-D volume of radiation-sensitive medium is also read by process of two-photon interaction. A domain is so read upon such times as two radiation pulsesxe2x80x94one of which radiation pulses may also be a pulse otherwise used for writing and which two pulses taken together have a joint energy that aggregates a predetermined second energy level less than the first energy levelxe2x80x94again temporally and spatially intersect in the domain, interacting therewith. The radiation-sensitive medium is totally insensitive to radiation of this aggregate second energy level, as well as to the energy of either beam taken individually, to change in the slightest, let alone to change state. Accordingly, reading is non-destructive.
Considering the previous paragraph, one way of describing the response of the 2-P 4-D radiation memory during reading is to say that it is selectively transparent. Neither radiation pulse can xe2x80x9cseexe2x80x9d anything within the volume of radiation-sensitive medium save that it temporally and spatially intersects the other pulse, and then only to the time extent and over the spatial interval where such intersection satisfies the quantum-mechanical equations of two-photon interaction. Both pulses xe2x80x9cseexe2x80x9d the same thing (if the molecules of the radiation-sensitive medium are randomly aligned, as is normal) at their locations of intersection, and each is modified in the same way.
Note that in the two-photon interaction each and both radiation pulses are not modified in accordance with their individual characteristics (such as frequency, and energy), but are modified identically accordance with the two-photon interaction. Both read radiation pulses accordingly xe2x80x9cseexe2x80x9d the same thing, and both are commonly detected so as to permit real-time redundant checking of the correct operation of the 2-P 4-D radiation memory.
Remarkably for a radiation memory that already has two (2) separate and redundant read radiation outputs (each of which is independently detectable in an associated array of photodetectors or the like), there is yet further radiation output from the memory during reading. The two stable indices of refraction selectively assumable by the radiation-sensitive medium represent, in the preferred spirobenzopyran radiation-sensitive medium, two different isomeric molecular forms of this spirobenzopyran medium. During reading each intersection domain will selectively fluoresce dependent upon its pre-existing isomeric molecular form.
This incoherent fluorescence occurs at a separate wavelength and frequency from each of the selectively transmitted read light pulses. It is also detectable by photodetectors or the like. Although detectable at any angle to the volume, the intersection domains will be unambiguously resolved only along an axis of illumination. The fluorescent light emissions may be split out from the coherent, illuminating, read radiation pulses also transmitted along these illumination axis by beamsplitters and like devices. Note that, because the fluorescent light emissions travel in all directions (unlike the directional read radiation pulses), such beamsplitters can be located in the path(s) of either of both read radiation pulses in locations before the volume of radiation-sensitive medium. Because there is little problem with laser production of read radiation pulses sufficiently intense so as to overcome any losses in a beamsplitter, there is little, or no, problem to redundantly detecting the fluorescence along one or both illumination axis, providing yet another redundant detection of the data stored in the intersection domains.
Although unnecessary for operation as a memory, and although projected to be uncommon during normal usage of the 2-P 4-D radiation memory, each domain within the 3-D volume of radiation-sensitive medium can still further be erased (changed from its second to its first state). Erasure is again by process of two-photon interaction, particularly two-photon absorption. A domain is erased upon such times as two radiation pulsesxe2x80x94one of which radiation pulses may also be a pulse otherwise used for writing and/or reading and which two radiation pulses taken together have a joint energy that aggregates a predetermined third energy level that is greater than the first (and second) energy levelxe2x80x94temporally and spatially intersect within the domain, interacting therewith by two-photon absorption.
The radiation-sensitive medium within each intersection domain is responsive to radiation of this third energy level to change from its second to its first state. If the radiation-sensitive medium is already in its second state then it is unchanged by radiation of the third energy level. The radiation-sensitive medium is insensitive to the energy of either illuminating erase radiation pulse taken alone, and is nowhere within the entire 3-D volume is the radiation-sensitive medium changed in the slightest by either erase radiation pulse taken individually.
According to this operation, and these properties, the radiation memory of the present invention is deservingly called four-dimensional, or xe2x80x9c4-Dxe2x80x9d for at least two reasons.
First, and as previously explained, fluorescence from the intersection domains can be detected, preferably along one or both illumination axis, to provide yet another redundant detection of the data that is stored within the intersection domains. The fluorescent light output is at a different frequency and wavelength from either of the read radiation pulses (that are both selectively refracted in their transmission through the volume of radiation-sensitive medium by the pre-existing indices of refraction stored in the intersection domains). Because each frequency of light output is a separate dimension, the radiation memory of the third related application is xe2x80x9c4-Dxe2x80x9d.
Second, the (i) three-dimensional (3-D) volume of a medium that is sensitive to radiation in its absorption band to undergo an anomalous change in index of refraction, and (ii) the manner of radiatively defining, changing and detecting selected domains within the 3-D volume of radiation-sensitive medium by process of two-photon (2-P) absorption, together also constitute a fourth dimension (4-D) to a standard 2-P 3-D radiation memory. Together they serve to make a radiation memory four dimensional (4-D) in a way that the inducement of selective changes in isomeric molecular form by process of two-photon absorption that is taught within the first two related predecessor patent applications do not.
To understand why this is so, and the second reason why the radiation memory of the third related patent application is deservedly called four-dimensional (4-D) rather than three dimensional (3-D), consider the following. Firstxe2x80x94and although any radiation pulse or beam will be somewhat affected by passage through a great length of radiation sensitive mediumxe2x80x94a single radiation pulse is substantially unaffected by any and all domains of varying refractive index through which it passes so long as it does not undergo interaction with any domain or domains by process of two-photon absorption. Indeed, the single radiation pulse is likely totally unaffected within measurement limits during its passage through short, fractional meter, lengths of a radiation sensitive medium.
Accordingly, if the dimension of a three-dimensional (3-D) volume of radiation-sensitive medium having domains that exhibit varying indices of refraction is kept quite smallxe2x80x94say on the order of one centimeter (1 cm.)xe2x80x94then a single radiation pulse (or beam) of less than threshold energy will pass through the medium substantially totally unaffected in any way, and will incur refraction if, when and wherexe2x80x94and only if, when and wherexe2x80x94it both spatially and temporally intersects a second radiation beam of appropriate energy. One way of regarding this phenomena is to consider that the radiation-sensitive medium is transparent to the single radiation pulse or beam save only where, and when, it is selectively rendered non-transparent by a temporally and spatially intersecting pulse or beam (of appropriate energy-.
Consider also the involvement of time. A radiation pulsexe2x80x94whether used for writing or for reading or for erasingxe2x80x94interacts with the radiation-sensitive medium only at the particular location(s) where it is both spatially and also temporally coincidence with an intersecting pulse. The two pulses affect, and are most substantially affected by, the radiation-sensitive medium not only at the location(s) of their spatial, but also of their temporal, intersection. This means that both radiation pulses both modify, and are themselves modified, in a temporal (time), or fourth, dimensionxe2x80x94as well as in the three spatial dimensions.
The 2-P 4-D radiation memory of the third related application makes use of its temporal, or time, dimensionxe2x80x94which is another reason why it is so named xe2x80x9c4-Dxe2x80x9dxe2x80x94in the addressing of domains within the 3-D volume of radiation-sensitive medium.
Consider reading. Each of two mutually intersecting radiation pulses exiting the volume of radiation-sensitive medium will bear the record of the refractive index of only the domains in which each pulse has intersected the other during its passage. Each pulse passes through the volume of radiation-sensitive media at near light speed, typically some substantial fraction of 3xc3x97108 meters per second. During the course of its passage each pulse intersects the other in sharply defined and located regions called domains. The volume (size) of the domains is set by (i) the speed of the pulses in the radiation-sensitive medium, and (ii) and the quantum mechanical requirement that the two pulses must be spatially and temporally coincident for a sufficient time and space so as to interact with the radiation-sensitive medium.
Curiously, and beneficially, the duration of a pulse need not be so short as the time it takes to traverse a domain that the pulse (and its companion pulse) serve to define. Consider that if each radiation pulse is traveling at some substantial portion of 3xc3x97108 meters/second, and if the volume of radiation-sensitive medium is on the order of 1 cm3, and if this volume is divided into 2xc3x97103 by 2xc3x97103 by 2xc3x97103 (i.e., 8xc3x97109) domains, then, if half the distance in each of three co-ordinate directions is devoted to domains (i.e., one-eighth the volume), then the dimension of each domain will be about 2.5xc3x9710xe2x88x927 meters and each beam will traverse this distance in about 8 femtoseconds (8xc3x9710xe2x88x9215 seconds). Laser pulses this short can be generated, but only with difficulty. Luckily, however, the quantum-mechanical equations of two-photon absorption require that each of two pulse should be considerably longer than 8 femtoseconds if it is to react by process of two-photon absorption over a distance of 2.5xc3x9710xe2x88x927 meters. In fact, each radiation pulse has a quite manageable length of about L0 picoseconds (10xc3x9710xe2x88x9212 seconds). Each intersection domain has a dimension of about (2.5xc3x9710xe2x88x925 meters)3, and 2xc3x97103 by 2xc3x97103 by 2xc3x97103 such domains (i.e., 8xc3x97109 total domains) fit within a three-dimensional volume of one cubic centimeter (1 cm3) with as much spacing in any co-ordinate direction between adjacent domains as the domains extend in that direction. (A xe2x80x9csafetyxe2x80x9d margin this great, or 100% margins, is not required, but the radiation-sensitive photochromic medium in its polymer matrix is exceedingly inexpensive, and there is little need to tightly pack the domains.)
As a final step to selecting one multiplicity of domains to be read, written, or erased during any one cycle, out of the very great number of domains that are within the entire volume, one of the pulses is variably delayed relative to the other pulse. If the radiation pulses intersect at other than 180xc2x0 (i.e., counter-propagating), and, for example, if the radiation pulses intersect at. 90xc2x0 (i.e., perpendicular) then one pulse must be so variably delayed in each of various regions of its planar wavefront to a different degree.
The present invention is usable with the 2-P 4-D optical memories of the third related patent application. Namely, it is usable in an volume radiation memory that is addressed by intersectingxe2x80x94particularly including counter-propagatingxe2x80x94pulses that are of (i) a particular time duration, and (ii) phase. In such a memory domain definition and addressing is no longer by light steeringxe2x80x94which is relatively difficult and often accomplished by relatively slow electro-optic devices that may even have mechanical movementxe2x80x94but rather by such laser light timing and phasing as can be done very well and quickly circa 1995 (witness fiber optic communication). Namely, the present invention is usable inxe2x80x94and indeed, preservesxe2x80x94a volume radiation memory where radiation is passing through unaddressed regions without altering or interacting with these regions in any substantial way, and where even the fluorescence as may be produced by addressed domains upon read interrogation is harmless to corrupt other domains of the volume memory.
2.2.4 Spatial Light Modulators
The three- and four-dimensional optical memories of the present invention do not depend upon spatial light modulators (SLMS) as their preferred arrayed light-encoding elements nor, for that matter, on Charge Coupled Devices (CCDs) as their preferred arrayed light-detecting elements. However, some preferred 3-D and 4-D embodiments of the present invention will be seen to employ spatial light modulators (as did the optical memories of the related predecessor inventions) and, because these advanced devices are still somewhat uncommon (unlike CCDs), their operation is explained in this BACKGROUND OF THE INVENTION section of this application.
A recent survey, circa 1990, of spatial light modulators is contained in the article Two-Dimensional Spatial Light Modulators: A Tutorial by John A. Neff, Ravindra A. Athale, and Sing H. Lee, appearing in Proceedings of the IEEE Vol. 78, No. May 5, 1990 at page 826. The following summary is substantially derived from that article.
Two-dimensional Spatial Light Modulators (SLMs) are devices that can modulate the properties of an optical wavefrontxe2x80x94such as the properties of amplitude, phase, or polarizationxe2x80x94as a function of (i) two spatial dimensions and (ii) time in response to information-bearing control signals that are either optical or electrical. SLMs usefully form a critical part of optical information processing systems by serving as input transducers as well as performing several basic processing operations on optical wave fronts.
SLMs, although once considered simply as transducers that permitted the input of information to an optical processor, have a broad range of applications, and are capable of performing a number of useful operations on signals in the optical domain. Some of the more important functions that have been demonstrated with SLMs are: analog multiplication and addition, signal conversion (power amplification, wavelength, serial-to-parallel, incoherent-to-incoherent, electrical-to-optical), nonlinear operations, and short-term storage.
The functional capabilities of SLMs can be exploited in a wide variety of optical computer architectures. Applications of 1-D and 2-D SLMs encompass just about ever} optical signal processing/computing architecture conceived.
SLMs may be classified as to type. The major classification categories result from (i) the optical modulation mechanism, (ii) the variable of the optical beam that is modulated, (iii) the addressing mode (electrical or optical), (iv) the detection mechanism (for optically-addressed SLMs), and (v) the addressing mechanism (for electrically-addressed SLMs).
The modulation of at least one property of a readout light beam is inherent in the definition of an SLM. Hence the first major category of SLMs is based on modulation mechanisms. The modulation mechanism employs an intermediate representation of information within a modulating material. An information-bearing signal, either optical or electrical, is converted into this intermediate form. The major forms of conversion mechanisms that are employed in 2-D SLMs are
(a) Mechanical
(b) Magnetic
(c) Electrical
(d) Thermal.
Of these conversion mechanisms, the electrical mechanism will be seen to be preferred for use in the three-dimensional optical memory of the present invention. In the electrical conversion mechanism, the electric field interacts with the modulating material at several levels, giving rise to different effects. The interaction can take the form of distorting the crystal lattice, changing the molecular orientations, or modulating the electron density functions.
A conversion mechanism and the modulating material so converted have a characteristic response time, activation energy, and spatial scale. These parameters, in turn, have a major impact on the respective speed sensitivity and spatial resolution of the optical modulation performed by the SLM. A modulation mechanism, however, becomes physically more specific only when combined with a choice of appropriate modulation variables, to be discussed next.
An optical wavefront has several associated variables that can be modulated as a function of the spatial coordinates and time in order to carry information. These variables include
(a) Intensity (amplitude)
(b) Phase
(c) Polarization
(d) Spatial frequency spectrum (texture).
Intensity (amplitude) and phase are the most commonly used representations in an optical computing system. Polarization and spatial frequency spectrum are often used as intermediate representations, and are converted into intensity or phase modulation before the information is used in the next stage of the optical computing system. Intensity (amplitude), phase, and polarization modulation will each be seen to be employed in the three-dimensional optical memory of the present invention.
Intensity, or amplitude, modulation commonly results when the absorption characteristics of a modulating material are changed. Because the intensity of a light beam is proportional to the square of its amplitude, the difference between these two modes depends on the variable that is employed in subsequent processing of a SLM output. The present invention will be seen to be more concerned with selectively controllably spatially modulating to zero intensity, and amplitude, then with any requirement that modulation at and to an opposite binary state should produce sufficient intensity, and amplitude, so as to permit a desired operation within an optical memory. This is because any presence of light intensity, or amplitude, in those spatial locations of an optical wavefront (i.e., at a particular time) where, and when, there is desirably no light intensity, nor any amplitude, constitutes optical noise.
The three-dimensional optical memory in accordance with the present invention will be seen to be innately highly insensitive to optical noise, being roughly sensitive to (noise/signal)2, as opposed to the lesser figure of merit noise/signal, in certain operations. Nonetheless to this innate insensitivity, optical noise may be cumulative in degrading the integrity of informational stores within the optical memory over billions and trillions of read and write cycles. Accordingly, intensity, or amplitude, modulation in accordance with the present invention is desirably very xe2x80x9cclean,xe2x80x9d with minimal, essentially zero, optical intensity or amplitude in those wavefront regions which are spatially modulated to one (xe2x80x9c0xe2x80x9d) binary state. Spatial light modulation, and SLMs, will be seen to so operate in the present invention: veritably no light will be in regions where it is not wanted.
Polarization modulation is commonly achieved by modulating the bi-refringence associated with the modulating material of the SLM. Bi-refringence is a property of some materials in which the refractive index depends on the state of polarization and direction of light propagation. Depending upon the effect utilized, the state of polarization changes (e.g., from linear to elliptical), or the angle of the linear polarization changes without changing the state of polarization. The memory system of the present invention will be seen to use phase-modulating SLMs that produce each such effect.
Polarization modulation can be changed into intensity (amplitude) modulation by employing polarized readout light and an analyzer in the output. The memory system of the present invention will later be seen to be so change polarization modulation into intensity modulation. Indeed, this will be seen to be a primary approach by which the net effective intensity, or amplitude, modulation will be rendered exceptionally xe2x80x9cclean,xe2x80x9d and of satisfactory quality to support reliable operation of volume optical memories over great periods of time and astronomical numbers of read and write cycles.
2.2.5 A Figure of Merit for a Readable and Writable and Erasable Optical Memory
Most new memory technologies are typically immediately gauged by the figures of merit that have attended past technologies. These previous figures of merit, while generally representing criteria that must be met by any operational computer memory, are often substantially irrelevant to the truly critical performance aspects, and to new figures of merit, that are appropriate to a new technology.
For example, the Intellectual Property Owners, Inc. gives annual awards in the name of its educational subsidiary the IPO Foundation to distinguished inventors. In the 1989 awards, Robert P. Freese, Richard N. Gardner, Leslie H. Johnson and Thomas A. Rinehart were honored for their improvements in erasable, re-writable optical discs introduced by the 3M Company during 1988. The optical discs can store 1,000 times as much information as conventional flexible diskettes used with personal computers. The inventors were the first to achieve a signal to noise ratio for an erasable optical disc in excess of 50 decibels.
Although the inventor of the present invention would be the first to recognize this contribution, and to acknowledge the necessity of an adequate optical (and electrical) signal-to-noise ratio for optical memories, a focus on signal-to-noise as a figure of merit may be rooted in the importance of this measurement in certain previous electrotechnology. For example, certain magnetic memories, such as stripe domain garnet film and Block line magnetic memories, have undesirably small signal-to-noise ratios.
It is uncertain what constitutes the ultimate, or even the most appropriate, figure of merit (or figures of merit) for a readable and writable and erasable optical memory. However, it is suggested that, in the case of a three-dimensional optical memory, it is important to consider whether or not, and how fast, the memory might become xe2x80x9cdirtyxe2x80x9d from use and suffer degradation in the integrity of its data stores.
The concept of a xe2x80x9cdirtyxe2x80x9d three-dimensional optical memory arises because every read and write operation on the memory by use of radiation has the potential to perturb other storage domains than just those domains that are intended to be dealt with. The most analogous prior memory technology may be the original square loop ferrite magnetic core memories. In these early core memories many millions of interrogations of one memory location had the potential of causing a single magnetized core having a weak hysteresis to fail to provide a sense signal adequate for detection of its magnetic condition, meaning the binary data bit stored. The affected bit was xe2x80x9cdroppedxe2x80x9d, or xe2x80x9clostxe2x80x9d. Even more relevantly, unaddressed and/or unwritten cores, commonly in physically proximate positions, may sometimes inadvertently and erroneously change hysteresis states causing attendant loss of data. The affected bit xe2x80x9cchangedxe2x80x9d.
A three-dimensional optical memory is analogous. The radiation that is used to read and write selective domains of the memory can, if great care is not employed, end up, after millions or billions of cumulative cycles, changing domains other than those domains that are desired to be changed. Such an undesired change of domains degrades the integrity of the data stored within the memory. The sensitivity of optical memories to degradation may be appreciated when it is understood that a single information may at present be stored within only a few hundred molecules (which photochromic molecules are likely within an inert matrix of many thousands more molecules per bit). Ultimately, a single bit of information would desirably be stored on a single molecule, although such a single molecule must, due to a diffraction limited spot size, be selectively addressed by some other means than the directed focusing of radiationxe2x80x94or at least radiation of the wavelength of light.
Accordingly, the present invention concerns not only addressed reading and writing and erasing a volume optical memories, and so doing at impressive levels of performance, but doing so by design, at a high figure of merit. A xe2x80x9chigh figure of meritxe2x80x9d means that an optical memory constructed in accordance with the present invention is practically and reliably useful in the real world, reliably storing and reading any and all data patterns with absolute integrity during indefinitely long periods of any pattern of use, or non-use, whatsoever. Consider that three-dimensional optical memories, storing information in a volume that is little more than a cube of plastic, are intrinsically physically amorphous and homogenous. It is prudent to use some care, and forethought, in the manner of radiative reading and writing of such a volume so that those changes that are selectively induced within selected domains of the volume should be absolutely stable and independent. Nothing should be done, or repetitively done, on any selected domains that adversely affects the integrity of non-selected domains.
An color optical memory of any dimension in accordance with the present invention will so function.
Moreover, the present invention will be seen to transcend, in a broad and substantial way, the previous barrier that a molecule or a group of molecules must be radiatively addressedxe2x80x94as an addressable domainxe2x80x94by bringing appropriate radiation beam or beams, pulse or pulses, to bearxe2x80x94both spatially and temporallyxe2x80x94on only a selectively addressed molecule or group of molecules. The present invention will be seen, in fact, to temporally produce the exact same radiation conditions at the spatial locations of many individual photochromic molecules, or groups of photochromic moleculesxe2x80x94containing many correspondingly discrete bits of informationxe2x80x94at one and the same time. The addressable domains are this xe2x80x9cextendedxe2x80x9d for containing more information than may be, by only spatial and temporal selection, addressed or isolated. However, in accordance with the present invention, (i) multiple individual photochromic molecules, or groups of photochromic molecules, that are different one to the next and that are co-located in the same spatially and temporally addressable domains, (ii) will nonetheless be seen to be selectively individually addressable. In other words, the addressing of different photochromic molecules, or groups of molecules, well next be seen to transcend diffraction limits, and yes, for the first time, it will be theoretically possible to manipulate a single molecule that is densely packed in a three-dimensional with trillions of its siblings and cousins, and even to detect a single photon that is representative of a binary state of this molecule. Accessible information storage at the molecular level is at present performed only by biological systems.
The present invention contemplates either a three- or four-dimensional radiation memory that serves to store multiple binary bits of informationxe2x80x94typically about five to ten and more typically eight such bitsxe2x80x94in the same physical volumes of each of a great multiplicity of addressable domainsxe2x80x94typically about 106 such domains per cm2 in each of potentially multiple layers within the entire volume of a planar disc, or, more importantly, from 1012 to 1015 such domains per cm6 in a random-access volume radiation memory. The storage of multiple information bits within the same addressable domains is enabled by the co-location of several different photochromic chemicals in the volume of each such domain.
(For an explanation of what precisely is meant by a xe2x80x9cfour-dimensional, or xe2x80x9c4-Dxe2x80x9d, volume memory, see the aforementioned co-pending U.S. Patent Application, and/or section 2.2.3 of the BACKGROUND OF THE INVENTION SECTION of this specification. Basically, a 4-D memory is a 3-D memory that is addressed by the use of time, as well as space, as a managed dimension of the addressing process.)
Each of the multiple information bits within each domain is capable of being selectively written (and may optionally be selectively erased) in a process of two-photon (xe2x80x9c2-Pxe2x80x9d) absorption induced by a pair of radiation beams, or pulses. Each pair of these write radiation beams, or pulses, is uniquely associated, a particular photochromic chemical of the several such that are within each addressable domain. Each pair of write radiation beams, or pulses, possesses an appropriate combined frequency and energy (i.e., a xe2x80x9ccolorxe2x80x9d) suitable to cause only one particular photochromic chemical to change. Accordingly, a single type of photochromic chemical, and a bit, of only one xe2x80x9ccolorxe2x80x9d is written at one time. (In certain particular combinations of photochromic chemicals, and of writing radiation beam pairs, it is theoretically possible to write a limited small number of different bitsxe2x80x94typically only two or so such different bitsxe2x80x94at the same time. However, it is not preferred that the xe2x80x9ccolorxe2x80x9d radiation memories of the present invention should be so used.)
All the multiple information bits stored within each domain are typically read (and are normally erased) in common and at the same time by the simple expedient of radiatively inducing all the different photochromic chemicals that are in an addressed domain to simultaneously fluoresce. This inducing of fluorescence is again by process of two-photon (xe2x80x9c2-Pxe2x80x9d) absorption, as is now resultant from a single pair of read radiation beams, or pulses, having an appropriate frequency, and corresponding combined energy, that are suitable to read all the bits that are stored in and by all the several different photochromic chemicals that are co-located within the volume of the addressed domain.
The fluorescence of each of the different photochromic chemicals upon readingxe2x80x94which fluorescence is selective in accordance with the written state of each such photochromic chemicalxe2x80x94is separated from, and is separately detected from, the fluorescence of all other photochromic chemicals. This is possible because the fluorescent light emission of each photochromic chemical is of a different color, and is spatially steered to an associated detector array by a color-sensitive beamsplitting mechanism as elementary as a simple prism.
(Again, it is theoretically possible to radiatively read some one or ones of the multiple photochromic chemicals, and the information bits stored thereby, more pronouncedly, and even in isolation, from the radiative reading of other photochromic chemicals storing other bits. In certain configurations of a color memory apparatus of the present invention such as, for example, an optical disc sweeping under multiple read stations each of which uses an appropriately xe2x80x9ccoloredxe2x80x9d read beam, a selective reading of individual xe2x80x9ccolorsxe2x80x9d is indeed contemplated. However, it is generally not preferred that a xe2x80x9ccolorxe2x80x9d radiation memory in accordance with the present invention should be so read piecemeal, and only some few xe2x80x9ccolorsxe2x80x9d at a time, but rather that all the xe2x80x9ccolorsxe2x80x9d of the memory should be read simultaneously.)
The read radiation beams of each color fluorescent light are preferably steered to a dedicated detector, normally an inexpensive Charge Coupled Device (CCD) detector array. (At the cost of a slower reading speed, it is possible to move a single detector into successive positions such as permits each color to be successively read,. It is even possible to forego splitting the multi-colored fluorescent light beam at all, and to simply rotate filters transmitting various colors (with attenuation) in a beam line on the way to a single detector or detector array. Generally, however, both monochromators, particularly prisms, and CCDs being inexpensive circa 1995, it is preferred to detect, as well as to read out, all the several colors of the color radiation/optical memory in parallel.)
Exemplary fluorescent photochromic chemicals suitable for simultaneous use in a color radiation/optical memory in accordance with the present invention include spirobenzopyran, rhodamine, cumarin and anthracene. Suitable groups of photochromic chemicals are desirably selected from individual species of photochromic chemicals exhibiting (i) narrow, sharp, separate spectra of absorption and emission that are (ii) suitably distinct from each other, where (iii) no fluorescent emission energy of any species overlaps the absorption energy of any other species.
According that a radiation/optical memory of the present invention is written and read with radiation of different frequencies, or colors, by a process of two-photon absorption, and that it produces fluorescent radiation of different frequencies, or colors, it is called a xe2x80x9ctwo-photon color radiation/optical memoryxe2x80x9d. The color radiation/optical memory operates to permissively store several, typically five to ten, bits of information in each domain of its entire volume. It does so by virtue of containing several different, but interrelated, photochromic chemicals dispersed and co-located throughout its entire volume, normally as a continuum. The addressable domains within the volume are not differentiated physically, but are defined by the two-photon addressing process as explained in the related predecessor patent applications.
The 2-P color radiation/optical memories of the present invention are improved over previous 2-P radiation/optical memories (i) for storing more information in the same volume (i.e., for exhibiting increased storage density), and (ii) for radiatively reading more information per unit time (i.e., for exhibiting an increased data readout rate). The magnitude of the improvement is roughly in accordance with the number of different photochromic chemicals that are present, and the corresponding number of bits that are stored, per addressable domain. The improvement is typically on the order of times five to times ten (xc3x975 to xc3x9710).
Moreover, any or all of the multiple bits stored in each domain of a 2-P color radiation memory are susceptible of being written, or read, or erased along radiation paths that arexe2x80x94save for such two-dimensional (2-D) spatial encoders of the beam(s) wavefront(s) as are used in writing, and such 2-D detector arrays as are used in readingxe2x80x94substantially coincident from one path to the next. Accordingly, a good deal of the structure, and the cost, of 2-P color radiation/optical memoryxe2x80x94which memory must function so as to direct radiation, typically laser light, beams to selected temporal and spatial coincidence at selectively addressed domains within its three-dimensional volumexe2x80x94is normally in common for at least the writing, and optionally also for the reading, of all the several binary bits that are stored in each domain. Accordingly, the complexity and cost of the memory does not increase in proportion to its increased density and readout performance.
The present invention is useful of incorporation in substantially planar optical media, potentially optical tape but more typically optical discs. A mere factor of five or ten (xc3x975 to xc3x9710) in (i) information storage density, and also (ii) in information readout speed, is not anticipated to appreciably alter the competitive balance between state-of-the art optical and magnetic media circa 1995. However, it must be realized that the entire annulus of an optical disc may be read through a lens to a massively parallel detector array (a CCD, or multiple CCD arrays in the case of the present invention) as is taught in U.S. Pat. No. 5,285,3438 for a MOTIONLESS PARALLEL READOUT HEAD FOR AN OPTICAL DISC RECORDED WITH ARRAYED ONE-DIMENSIONAL HOLOGRAMSxe2x80x94producing gigabyte data transfer rates from hundred dollar level components.
Optical discs may well be xe2x80x9cdragging a lot of baggagexe2x80x9d, and may be somewhat impacted in their deployment, circa 1995 because many persons who clearly understand the operating principles and well-established uses of the Edison gramophone phonorecord, the magnetic disc, and the CD-ROM have mentally consigned all discsxe2x80x94including optical discsxe2x80x94to the class of single-head serial-access devices having an irreducible latency in data access. Resultant to such mental classification, certain persons have virtually ceased all creative thinking about optical discs not directed to making them reliably re-writable. It is suggested that any implementation of a radiation memory in the form of an optical disc should be considered as a method of bringing the (rotating) radiation-sensitive medium spatially into position relative to a laser light, and spatially into position for its light output(s) to be detected, as opposed to the complimentary methodxe2x80x94mandated for a stationary 3-D optical memory mediumxe2x80x94of bringing the laser light (by light steering, which is difficult) into spatial position onto the medium. Once this mode of thinking is adopted, it becomes increasingly clear that an optical disc can assume some of the advantages of a full 3-D or 4-D volume radiation/optical memory.
In general, however, the present invention comes into its full glory in 3-D and 4-D volume radiation memories as are taught in the related patent applications. The ultimate preferred implementation of the color radiation memory of the present invention is as a 2-P 4-D color radiation memory, meaning a memory as taught in the third related patent application where the addressing of the domains within a 3-D volume of photoactive media is by control of the relative timing, meaning the phase relationship, of two spatially-intersecting radiation pulses, and does not transpire solely by any steering of these intersecting pulses. The radiation pulses are typically laser light pulses.
In accordance with the fact that (i) laser light may be readily turned on and off quickly so as to be impressed with binary information, and (ii) the two pulses of laser light temporally and spatially intersect within a diffraction-spot-size-limited domain of minuscule size during but an very short time interval (that is properly a complex function of the overlapping waveforms of the two beams), very small domains, typically on the order of 5xc3x971010 such domains per cm3 (see FIG. 8b and accompanying text) within the 2-P 4-D color radiation memory may be reliably and accurately addressed very quickly. (However, in accordance that the intersection xe2x80x9cdwellxe2x80x9d time of the intersecting laser light pulses at each domain is very short, and in accordance with the quantum mechanical equations of two-photon absorption, each laser beam must normally be quite bright for the photochromic chemicals of interest.) Nonetheless that each addressable domain is already quite small, it will, in accordance with the principles of present invention, contain a plurality of photochromic chemicals each of which stores an associated binary data bit. Among all the plural bits that are stored in each addressable domain, each bit is normally written at a separate time. This is accomplished by two radiationxe2x80x94normally laser light beamsxe2x80x94that are of an energy suitable to change the statexe2x80x94normally the isomeric molecular formxe2x80x94of a selected one of the plural photochromic chemicals while not appreciably altering any of the other, un-selected, photochromic chemicalsxe2x80x94even at the intersection domains.
However, reading functions oppositely. Any or all of the plural bits stored in each domain are susceptible of being read at the same time. Reading of information is by detection of (two-photon-induced) fluorescence, which is at a characteristic color for each of the different photochromic chemicals. Each read fluorescent radiation emission of a different color that exits the memory is steered, preferably by a simple diffraction grating, to a corresponding detector array.
The simultaneous readout of the 2-P 4-D color optical memory (and of a 2-P 3-D color optical memory) is particularly useful for three-dimensional (3-D) color television. If the information stored in the memory is of the three, or four, color separates of a color image, and if the color fluorescence of the readouts are related in frequency (i.e., in color) to the information stored, then all the read fluorescent radiation emissions of different colors may be viewed directly, or indirectly through color filters, by the eye as a color image. Reading of successive bit planes within the memory presents successive images to the eye in the manner of television. The presentation speed of the images easily exceeds the flicker fusion frequency (70 Hz) of the human eye. A 2-P 3-D, or 4-D, color radiation memory in accordance with the present invention therefore not only serves to store multiple bits of information in the same addressable physical domains, but is capable of supporting the temporally simultaneous, parallel, reading of all the information stored in these domains.
Accordingly, the present invention may be characterized by its embodiment as a radiation memory apparatus for storing binary information by radiation. This embodiment includes a matrix of a radiation-transparent stable material.
A number of different photoactive media, or photochromic chemicals, are contained and distributed all together inside, and as a continuum within, the matrix. Each individual medium transitions from a first to a second stable form in response to receipt of radiation falling within a transition energy spectrum. The transition energy spectrum of each medium unsubstantially overlaps the corresponding transition energy of any other ones of the media.
A first source of radiation first-radiates with a first beam of first-selected-energy radiation at least a selected portion of the matrix, and all the plurality of photoactive media contained within this portion. This first-radiating excites some one or ones, including at least a selected one, of the plurality of photoactive media that are contained within this portion. The excitation changes each susceptible medium from a first state to a new virtual state.
A second source of radiation second-radiates with at least one second beam of second-selected-energy radiation at least a selected part of the selected portion of the matrix, and of the plurality of photoactive media contained within this selected part. This second-radiating is simultaneous with the first-radiating in order to change the selected one, only, of all the plurality of photoactive media that are within this selected part of this selected portion from its virtual state to its second stable state by process of plural-photon absorption.
Notably, the combined energies of the first-selected-energy radiation and the second-selected-energy radiation jointly fall within the transition energy spectrum of the selected one of the plurality of photoactive media. Moreover, these combined energies of the first-selected-energy radiation and the second-selected-energy radiation jointly cause the selected part of the selected portion of the selected one of the plurality of photoactive media to change from its first to its second stable form by process of plural-photon absorption.
Notably however, the combined energies of the first-selected-energy radiation and of the second-selected-energy radiationxe2x80x94although sometimes jointly exceeding a transition energy of one or more of un-selected ones of the plurality of photoactive mediaxe2x80x94are substantially ineffectual to jointly cause any change in these un-selected ones. This is because there is an insubstantial overlap of the transition energy spectra of these un-selected ones of the plurality of photoactive media with the transition energy spectrum of the selected one of the plurality of photoactive media!
According to this operation but one single photoactive medium out of all the plurality of photoactive media that are all contained and distributed together in the matrix as a continuum will be written.
This preferred embodiment of a radiation memory apparatus for storing binary information may be a substantially planar volume (e.g., a tape, or a card, or a disc) or a three-dimensional volume (e.g, a cube).
The preferred apparatus is, for use as a memory, further capable of radiatively readingxe2x80x94as stored binary-stated informationxe2x80x94the changed state of the selected part of the selected portion of the selected one of the plurality of photosensitive media.
The preferred apparatus for so doing includes a first read radiation source for third-radiating with a third beam of third-selected-energy radiation at least a selected portion of the matrix, and all the plurality of photoactive media contained within this portion. Similarly to the first-radiating (during a write operation), this third-radiating excites some one or ones, including at least a selected one, of the plurality of photoactive media that are contained within this portion, each from a first state to a new virtual state.
Simultaneously, a second read radiation source fourth-radiates with at least one fourth beam of fourth-selected-energy radiation at least a selected part of the selected portion of the matrix, and of the plurality of photoactive media that are contained within this selected part.
Notably, the simultaneous third- and fourth-radiating will cause all the plurality of photoactive media that are within this selected part of this selected portion to fluoresce by process of plural-photon absorption. Each will, however, do so only at an individually associated frequency, meaning in an individually associated colored light.
A monochromator in the form of a prism, a diffraction grating, or a like frequency-sensitive light steering device serves to spatially separate the color fluorescence as arises at all the plurality of photoactive media (that are within the selected part of the selected portion). This separating is in response to the colors of the colored-light readout beams.
A number of detectors, normally so many as there are colored-light readout: beams, detect as binary-stated information the changed states of the selected one, and all other ones, of the plurality of photosensitive media that are within the selected part of the selected portion.
Accordingly, all the binary-stated information that is stored within all the changed states of all the plurality of photosensitive media that are within the selected part of the selected portion is radiatively read at the same time. Nonetheless to being so collectively read, the binary-stated information as is uniquely associated with the changed state of any individual one of the plurality of photosensitive media is uniquely and unambiguously detected.
These and other aspects and attributes of the present invention will become increasingly clear upon reference to the following drawings and accompanying specification text.